Many conventional poly-phase power systems utilize some means to protect the system against faults. Faults, such as line-to-line and line-to-ground faults, can cause considerable damage to power system equipment, and as such, protection against faults is desirable. The results of faults can include fires, smoke, and melted or vaporized power system components. Further, due to the prospects of, for example, aging wiring in civilian and military aircraft, and the reduced thickness of insulation of modern wires, the likelihood of faults on power systems is increasing.
Faults can be low-impedance or high-impedance. In the presence of low-impedance faults, current flow on a system can increase substantially, far exceeding the normal load current on the system. As such, current sensors are often used to detect low-impedance faults. On the other hand, high-impedance faults, such as arc faults, do not cause the same increases in current. Often high-impedance faults can generate current levels similar to or less than normal load current. As such, protective devices that merely monitor current can be unaware of a high-impedance fault condition. Accordingly, catastrophic damage to system components can result because the fault remains on the system for a substantial duration. In the case of an arc fault, the fault can often remain on the system until the fault burns clear. Since portions of arc faults can reach 6000 degrees Celsius, an arc fault burning clear can involve the vaporization of metal components, fires, smoke, etc. Accordingly, a means of detecting high-impedance faults, such as arc faults, is desirable.
Many issues challenge the performance of any fault detection scheme. First, the scheme must be able to recognize actual faults, dissipating only a fraction of full load power without also generating erroneous, non-fault related, tripping under normal load characteristics. Conventional protection devices, such as fuses, breakers, and the like, cannot meet this requirement because these devices only protect against currents exceeding full load. As such, when high-impedance faults generate less than full load current conventional protection devices do not react and the fault remains on the system. Second, a fault detection scheme must be able to detect and react to a fault quickly to minimize damage to power system components. Conventional time-over-current devices may take from seconds to minutes to operate, which can be too long to effectively protect system components. Applied Physics Laboratory (APL) and other institutions have determined that arc faults, in particular, should be cleared within 100 milliseconds to keep damage safely localized. Since interrupting contactors may take as long as 50 milliseconds to open, detection in the 20 to 50 millisecond range is desirable for an arc fault detection scheme.
Several conventional methods of protecting power systems against high-impedance faults, including arc faults, have been developed. For instance, some systems utilize arc fault circuit interrupters (AFCIs). AFCIs can employ proprietary algorithms to detect certain features of arc fault currents on DC and single phase AC circuits, such as spikes, dead zones at zero crossings, etc. AFCI protection schemes are primarily aimed at low power, repeating “snapping” or “ticking” type arcs, rather than high power sustained arcs. Another high-impedance fault detection scheme utilizes bifurcated wiring. In this scheme, every load wire is split into a pair of wires. If a fault occurs on one wire in the pair, the current on the faulted wire can be compared to the current on the other, unfaulted, parallel wire. A difference in the currents between the two wires indicates a fault is present on the power system. Although this scheme is simple and effective for both AC and DC circuits, splitting each load wire in two is not always practical. A third scheme does not use electrical waveforms at all. Instead, optical and pressure sensors can be used simultaneously to determine that an arcing event is in progress. The waveforms from these sensors are orthogonal in the sense that either waveform can occur during normal operations, but only an arc fault will produce both at the same time. This system is also effective for any type of AC or DC power system, but it introduces considerable complexity to a power system protection scheme. As a result, this scheme is unlikely to find use in, for example, an airborne application. Additionally, some electric utility companies use zero sequence and negative sequence relays to detect high-impedance faults. This scheme, however, requires at least two relays and current sensors, which are not always available and require regular maintenance to ensure accurate operation.
While numerous schemes can be utilized to detect power system faults, some focus on the harmonics generated when a fault in present on a power system. As such, when a high-impedance fault is present, conventional current sensing devices may not detect a change, but devices that are attentive to the harmonics on the power system can detect the presence of a high-impedance fault.
To assist in an explanation of fault induced power system harmonics, FIG. 1a. depicts an exemplary three-phase power system with a rectifier. The power system of FIG. 1a comprises an AC source 100, line impedances 110, a rectifier 120, and a rectifier output 130. FIG. 1b depicts the voltage waveforms that can be obtained at rectifier output 130 over a single cycle of the power system. Waveform 160 is a waveform obtained at rectifier output 130 when no fault is present on the power system. Waveform 170 is a waveform obtained at rectifier output 130 when an exemplary C-phase line-to-ground fault 140 is present on the power system. A C-phase line-to-ground fault can be demonstrated by closing a switch at 140. Finally, waveform 180 is the waveform obtained at rectifier output 130 when an exemplary A-phase to B-phase, line-to-line fault 150 is present on the power system. An A-phase to B-phase, line-to-line fault can be demonstrated by closing a switch at 150.
In a balanced unfaulted power system, the lowest harmonic present at rectifier output 130 is the sixth harmonic, which can be seen in waveform 160. When line-to-ground fault 140 is applied the power system, the voltage in the faulted phase, C phase, is depressed due to extra fault load, causing the ripple pulses corresponding to conduction in C phase to decrease. This can be seen in waveform 170. Since the ripple pulses occur repetitively twice per line cycle, a second harmonic component can be generated. Similarly, when line-to-line fault 150 is applied, the ripple pulses corresponding to conduction in both faulted phases, A phase and B phase, decrease heavily, whereas the ripple pulses corresponding to conduction in only one of the faulted phases decrease only slightly. Again, since this happens repetitively twice per line cycle, a significant second harmonic component is generated. Thus, second harmonic content in the output of a rectifier on a three phase system can serve as a fault indicator for both line-to-ground and line-to-line faults.
Further, the failure or removal of a diode in rectifier 120 will distort or eliminate two ripple pulses in succession during each line cycle. As such, a significant first harmonic component can be generated.
Note that the power system of FIG. 1a is a balanced system and as such, first and second harmonics arise only as a result of a faulted condition. In imbalanced power systems first and second harmonics can arise as a result of the imbalanced load. However, when fault conditions occur on an imbalanced power system, the first and second harmonic component of the waveform at the rectifier output will still increase. As such, in an imbalanced system, an imbalance reference value can be determined when the system is in a maximum normal load imbalance condition with respect to a predefined system frequency harmonic. Since harmonic content increases as a function of fault power, the imbalance reference value can be used as a threshold value for comparison purposes to determine if a fault has occurred on the power system. Thus, when harmonic content exceeds the imbalance reference value, the harmonic content can be attributed to a fault condition.
Further, the source impedance of an AC power system can affect the harmonic content of a system waveform under fault conditions. As source impedance increases, the disparity in the rectifier output ripple pulses during a fault likewise increases. As such, the magnitudes of the resultant harmonics increase as well.
FIG. 2 depicts a power system and a fault detection system that is integrated into a Humbucker active ripple cancellation feedback control scheme that utilizes first and second harmonics of the power system frequency to detect faults. Three-phase AC power source 200 feeds a breaker 210 and a power-train controllable, line-commuted rectifier 220, and a Humbucker active ripple cancellation feedback control with fault detection capabilities 230.
A conventional Humbucker active ripple cancellation scheme can be used to drive down to zero, a harmonic component of a waveform. The Humbucker scheme of FIG. 2 utilizes a high pass filter 240 and two synchronous band pass filters (SBPFs) 250 and 260 to achieve these results. The Humbucker active ripple cancellation feedback control with fault detection capabilities 230 utilizes a SBPF 250 and associated circuitry to detect first harmonics and an SBPF 260 and associated circuitry to detect second harmonics. When harmonic components are present in a power system waveform, the high pass filter 240 can be designed to take a rectified waveform from rectifier 220 and filter the low-frequency components, such as DC, from the waveform leaving only the components of the waveform that are of interest to fault detection, such as, first and second harmonics. The waveform can then be passed to SBPFs 250 and 260. Within each SBPF 240 and 250 are two pure integrators 270, which provide infinite gain at a predefined frequency, such as, for example, first and second harmonics of a power system frequency. The integrators 270 produce near DC signals that are proportional in magnitude to the magnitude of the predefined frequency. As the harmonic component of the waveform increases, the magnitude of the output from the integrators 270 will increase. The Humbucker uses these signals to ultimately drive the harmonic components out of the power system waveform, but these signals can also be used for fault detection. As such, the outputs of the integrators 270 can be compared to an imbalance reference value at comparators 280 to determine if a harmonic component of a waveform exceeds a reference which can indicate a fault condition on a power system.
While the Humbucker solution depicted in FIG. 2 can detect a first and second harmonic component of a power system waveform and use that information to detect faults, it does so only in a context where the harmonics on the power system are being driven down to zero by the Humbucker scheme. Thus, in applications where alteration of the power system waveform is undesirable, the Humbucker solution is inapplicable. As such, it may be advantageous to provide an improved mechanism for accurately and quickly detecting high-impedance faults on poly-phase power systems where the waveform of the power system is not altered as part of the fault detection scheme.